We are developing a rapid test for effectiveness of agents used in treating disseminated cancer. Cesium 131 has been shown to be actively taken up and concentrated by actively metabolizing cells, including tumor cells. Differential injury of susceptible tumor cells by drugs, hormones, or radiation reduces their intake of cesium compared to normal tissues in vivo in patients. The change in uptake of superficial metastases after about 3 days of treatment correlates well with later response to the same treatment. Improvement in accuracy is being sought by testing refinements of the method. Increased applicability of the test is expected with use of more powerfully radiating isotopes of cesium, namely Cs 130, Cs 129, or Cs 127, with which deep-lying tumors can be studied. A better knowlege of the effects of different treatments on tumors is being sought in study of frozen section cesium 131 radioautographs of murine cancer.